NAME:
  angsep
 PURPOSE:
  Compute the angular distance between two spherical coordinates.
 DESCRIPTION:

 CATEGORY:
  Mathematical
 CALLING SEQUENCE:
  ans=angsep(ra1,dec1,ra2,dec2)

 INPUTS:
  ra1  - RA of first position (radians)
  dec1 - Dec of first position (radians)
  ra2  - RA of second position (radians)
  dec2 - Dec of second position (radians)

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:

 OUTPUTS:
  return value is the angular distance in radians.

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  Written by Marc W. buie, Lowell Observatory, 1997/09/08
  2009/02/26, MWB, added protection against round off error generating NaN